Electrum is a naturally occurring alloy of gold and silver, with trace amounts of copper and other metals. It has also been produced artificially, and is often known as green gold. The ancient Greeks called it 'gold' or 'white gold', as opposed to 'refined gold'. Its colour ranges from pale to bright yellow, depending on the proportions of gold and silver.

The gold content of naturally occurring electrum in modern Western Anatolia ranges from 70% to 90%, in contrast to the 45–55% of electrum used in ancient Lydian coinage of the same geographical area. This suggests that one reason for the invention of coinage in that area was to increase the profits from seigniorage by issuing currency with a lower gold content than the commonly circulating metal.

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Electrum consists primarily of gold and silver but is sometimes found with traces of platinum, copper, and other metals. The name is mostly applied informally to compositions between about 20-80% gold and 20-80% silver atoms, but these are strictly called gold or silver depending on the dominant element. Analysis of the composition of electrum in ancient Greek coinage dating from about 600 BC shows that the gold content was about 55.5% in the coinage issued by Phocaea. In the early classical period, the gold content of electrum ranged from 46% in Phokaia to 43% in Mytilene. In later coinage from these areas, dating to 326 BC, the gold content averaged 40% to 41%. In the Hellenistic period, electrum coins with a regularly decreasing proportion of gold were issued by the Carthaginians. In the later Eastern Roman Empire controlled from Constantinople, the purity of the gold coinage was reduced, and an alloy that can be called electrum began to be used.

The colour of electrum is pale yellow or yellowish-white and the name is a Latinized form of the Greek word ἤλεκτρον (èlektron), mentioned in the Odyssey referring to a metallic substance consisting of gold alloyed with silver. The same word was also used for the substance amber, likely because of the pale yellow colour of certain varieties, and it is from amber's electrostatic properties that the modern English words "electron" and "electricity" are derived. Electrum was often referred to as white gold in ancient times, but could be more accurately described as "pale gold". The modern use of the term white gold usually concerns gold alloyed with any one or a combination of nickel, silver, platinum and palladium to produce a silver-coloured gold.

Electrum is possibly referred to three times in the Bible (i.e. if the Septuagint's translation of the uncertain term חַשְׁמַל is accurate). In all three instances it is used to describe a type of glow seen in visions by the prophet Ezekiel (Ezekiel Ch.1 Vs.4 and 27;Ch. 8 Vs. 2).

Electrum was much better for coinage than gold, mostly because it was harder and more durable, but also because techniques for refining gold were not widespread at the time. The discrepancy between gold content of electrum from modern Western Anatolia (70–90%) and ancient Lydian coinage (45–55%) suggests that the Lydians had already solved the refining technology for silver and were adding refined silver to the local native electrum some decades before introducing the pure silver coins cited below.

In Lydia, electrum was minted into 4.7-gram coins, each valued at 1/3 stater (meaning "standard"). Three of these coins (with a weight of about 14.1 grams, almost half an ounce) totaled one stater, about one month's pay for a soldier. To complement the stater, fractions were made: the trite (third), the hekte (sixth), and so forth, including 1/24 of a stater, and even down to 1/48th and 1/96th of a stater. The 1/96 stater was only about 0.14 to 0.15 grams. Larger denominations, such as a one stater coin, were minted as well.

Because of variation in the composition of electrum, it was difficult to determine the exact worth of each coin. Widespread trading was hampered by this problem, as cautious foreign merchants offered poor rates on local electrum coin.[citation needed].

These difficulties were eliminated in 570 BC when pure silver coins were introduced. However, electrum currency remained common until approximately 350 BC. The simplest reason for this was that, because of the gold content, one 14.1 gram stater was worth as much as ten 14.1 gram silver pieces.